High-power compressional wave radiator



Feb. l0, 1948. w, P, MASON 2,435,595

HIGH POWER COMPRESSIONAL WAVE RADIATOR original Filed Feb. 19, l1942 /la 1/24 93a 14a FIG. .3l v A v 6 cAsron au.

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' be' 4a /NvENroR W P MASON M2-ww i A TTURNEV v Patented En.. 1o, 194s .UNITED STATES PATENT l OFFICE HIGH-POWER COMPRESSIVONAL WAVE l RADIATOR' Warren P. Mason, West Orange, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application February 19, 1942, Serial No. 431,558. Divided and this application August 19, 1943, Serial No. 499,223

1 Claim. (Cl. 177-386) This invention relates to high power compressional wave radiating devices. More particularly, it relates to methods and apparatus arrangements for eliminating or substantially reducing the power limitations imposed upon compressional wave radiating devices by the phenomenon known as cavitation."

This application is a division of my copending application Serial No. 431,558, led February 19, 1942, which issued on July 23, 1'946, as United States Patent 2,404,391. K

When a vigorously vibrating mechanical element, such as a piezoelectric or magnetostrictive vibrating element, is employed in certain liquids, such as sea water, to radiate substantial amounts of compressional wave energy, it has been found that the amount of energy radiated increases directly with the increase in the power applied to the vibrating member up to a particular value,

which value varies somewhat with the specific liquid and with temperature variations of the liquid and the vibrating element. As the power. input is increased beyond this particular value,

' the phenomenon designated cavitation is observed.

This phenomenon is evidenced by the formation of bubbles of vapor or gas on the surface of the vibrating element and the amount of energy radiated per unit of further increasein energy applied to the vibrating element becomes much less than'for power levels below the cavitation point. In many instances, indeed, no increase in radiated power, or even a decrease in the total radiated power, will result from an increase in applied power once the phenomenon of cavitation has become evident.

This application is accordingly directed to the discovery that if the vibrating element isv immersed in a highly viscous fluid the phenomenon of cavitation does not.. become evident untl much higher power levels are reached.

For example, cavitation occurs in kerosene when the power input is suiilcient to create an acoustic pressure `of from .85 to .90 atmosphere at the surface of the vibrating member, whereas in highly viscous liquids, such as castor oil, olive oil. linseed oil, and the like, it has been found or members within a cap which is iilled with av highly viscous liquid, the cap being-appropriately shaped and proportioned to provide the desired spreading" of the power radiated from the vibrating member or membersover a. sufficientlylarge area before contact with and transmission into the sea water from the exterior of the cap takes place.

The principles of the invention are applicable with particular advantage to piezoelectric radiators employing Rochelle salt crystals, since, asdiscussed -at length in my copending application Serial No. 413,429v iiled October 3, 1941, entitled Compressional ,Wave radiators and receivers,

Patent No. 2,414,827, the heating of a Rochelle that cavitation does not take place until an acoustic pressure at the surface of the vibrating member in the order of tive atmospheres, or greater, has been established.

Since the radiated power is proportional to the square` of the acoustic pressure, the maximum output 'power which can be radiated by employing theA highly viscous liquid is in the order of salt crystal from any cause to a temperature of 40 C. or higher is most objectionable because it reduces the leakage resistance of the crystal .to such a low value that further heating and ultimate destruction of the crystal is very likely to result. Naturally the substantial dissipation of energy accompanying cavitation rapidly overheats the crystal and` accelerates its destruction.v

On the other hand, if this diiiiculty can be substantially eliminated Rochelle salt crystals are greatlyv to be preferred for submarine signaling compressional wave radiators since they are much vcheaper than other types of piezoelectric crystals and the natural frequency and impedance characterlsticsl of these crystals are more convenient for use in submarine signaling systems.

A principal object of this invention is to pro-l vide methods and apparatus for eliminating or reducing the phenomenon of cavitation" adjacent compressional lwaveradiatirxg surfaces.

Another object is to provide methods and apparatus for greatly increasing the power which can be efciently radiated from compressional wave radiators.

A further object is to provide'piezoelectric Rov chelle salt crystal radiators capable ofelciently radiating high power compressional energy Waves. Other objects willbecome apparent during the 3 course oi' thefollowing description and from the appended claim. l K Y The principles of the invention will be more readily apparent from the description of the illustrative embodiment shown in the accompanying drawing. in which:

Fig. 1 indicates in schematic diagram form the electrical connections of the radiator of Figs. 2 and 3: and

Figs. 2 and 3 show the principal mechanical features of a multicrystal piezoelectric high power compressional wave radiator employing the principles of the invention.

In more detail, in Fig. 1 a plurality of groups of four piezoelectric crystals in each group, namely, groups 1a, 2a, 3a, etc., are shown associated with a multisection band-pass electrical wave iilter comprising shunt arms I5, I'I, I9, etc., and series arms I6, I8, 20, etc. Successive groups of the crystals are connected electrically in shunt' with successive shunt arms of the nlter, respectively, as shown in Fig. 1. vThe right end of the ilter is terminated in a resistive impedance 26 which is appropriately related to the impedance of the adjacent lter section over the transmitting band of the latter as will be described hereinafter. If the arrangement is to be used as a radiator, electrical energy comprising frequencies within the pass-band of the filter is introduced through the terminals 32 at the left end of the filter. When used as a receiverthe compressonal wave energy is converted by the'crystals into electrical energy which may be drawn from terminals The crystal groups 1a, 2a, 3a, etc., are preferably aligned with a distance between centers which is less than half the wavelength of the highest frequency to be radiated or received.

'This is necessary in order that a single direction of transmission or reception will obtain for each frequency. As wavelength is the quotient of vetion of the type employed in the lter illustrated in Fig 1 is well known in the art, For example, see the textbook Transmission Networks and Wave Filters, by T. E. Shea, published by D. Van Nostrand Company, Inc., New York, 1929, pages 215 and 216, Figs. 106 and 107. It varies from 1r at the lower cut-off, to +viat the upper cut-off, passing through zero in the mid-frequency of its transmission band. Thus any desired phase shift, between the above-stated limits,

per filter section can be obtained by selecting the frequency in the pass-band corresponding to the desired phase shift. AOf course, for each partcular phase shift per section the array of crystal groups will transmit or. receive energy with greatest eiliciency at a particular angle since each crystal group diilers in phase from adjacent groups by the phase shift of one lter section.

If two or more diilerent frequencies, within the pass-band of the filter sections, are introduced into the device of Fig. 1, each frequency will be transmitted in a. particular direction, different for each frequency, respectively, since the phase shift per tllterv section will be diilerent for each frequency.

Conversely, for 'the reception of energy of a particular frequency within the pass-band oi' the illter sections the arrangement illustrated in Fig. 1 will respond with maximum eilciency only if the energy approaches from a particular angle which is dependent upon the phase shift per section of the wave filter at that frequency.

The arrangement of Fig. 1, therefore, normally has prismatic" properties as defined above.

Relay switches 28 are provided to operate on voltage placed on conductor 30 and to disconnect the ungrounded conductors ci the crystal groups from their respective filter sections and to connect them to a common conductor 3l so that all crystal units may be operated in phase at all frequencies in the event that prismatic characteristics are not desired. Radiation will then be broadside or normal to the longitudinal axis of the array for all frequencies.

Figs, 2 and 3 show the salient mechanical design features of a-device, the electrical schematic of which can be that shown in Fig. 1. To permit the radiation of greater power, and to increase the vertical directivity characteristics of the device, each. of the crystal groups la, 2a, 3a, etc., of Fig. 1 is further expanded by adding eight similar groups of four crystals each, connected electrically in parallel with the original group so that the complete radiator of Figs. 2 and 3 comprises fourteen vertical rows of crystal groups, each row comprising nine groups and each group comprising four crystals, i. e.. thirty-six crystals per row and ilve hundred and four crystals for the complete radiator. The nine groups of each row, for example, groups Ia to Ii, inclusive, or Ilia to iti, inclusive, as shown in Fig, 2, can conveniently have common electrodes (55 of Fig, 2) running the entire length of the row. Five electrodes are, of course, required, one between each pair of adjacent crystals and one on the outer surface of the two outside crystals of each group. The elecl trodes 55 can be of metal foil and are cemented or otherwise attached to the adjacent crystal surfaces.

The groups of crystals of each row are in turn cemented to their respective mounting strips 45, a thin insulating spacer being interposed between the crystal groups and the metal mounting plate to Aafford high insulation resistance. .Ceramic spacers of low dielectric constant and a cement also of low dielectric constant and unusually strong adhesive properties have been found most suitable for this purpose, since lower values of capacity to ground as well as higher breakdown voltages are thus obtained. These features are of especial importance for devices which are to radiate high power. The crystal groups are equally spaced along the mounting strip with a small interval between groups to ailord appropriate directivity in the vertical plane as described hereinafter. 'I'he mounting strips 45 are screwed to a mounting plate 42, which is assembled, as shown in Fig, 3, to clamp the edges of a composition rubber cover do tightly against a casing 52 by means of bolts 48 and nuts 50. The composition rubber cover is preferably of a material recently developed by one or more of the large rubber manufacturers to have substantially the same velocity of propagation of compressional wave energy as sea water and thus to increase the efllciency of energy transfer to or from the water.

A multisection lter. such as is illustrated in sans such lcrystals varies very little with temperaturev changes and will not,` therefore.. appreciably impair the lters characteristics with ltemperature changes normally encountered in submarine signaling. As a practical matter theprovision of small trimming capacities whereby the resonance of the arms may be exactly adjusted after final l assembly will be found advantageous.

Gaskets 5l of rubber or oil-proofed felt or the like are placed between the mounting strips 4 5- of adjacent rows and the space between the rubber cap and the crystals is nlled with castor oil or s ome other highly viscous fluid, such as olive oil or linseed oil, which will eliminatelcavitation at the power level to be employed and will serve to efiiciently transmit compressional wave energy. The viscous uid should be of such character that it can be dried conveniently to exc clude moisture from the crystal surfaces.

The mounting strips 45 include a backing block of metal 44 for each group of crystals. The

Y crystal groups are mechanicallyv one-quarter wavelength high andthe backing blocks are likewise mechanically one-quarter wavelength high, the diiferen'ce in height between the crystals and the backing blocks arising from the difference in the velocity ofpropagation of compressional wave energy in the two materials. This type of mounting is discussed in detail in my above-mentioned Yin my above-mentioned copending application.V

v copending application Serial No. 413,429 and its purpose is to produce a node of motion at the mounting strip 45 so that energy from the crystal groups in one row will not be transmitted to groups of adjacent rows or to the mounting plate noise from the craft itself will then be very likely to mask the relatively weak sound waves from a distant/submarine. Wiring between vthe crystals and illter, etc.. is not shown in Figs. 2 and3 as it would, it is felt, render the drawing obscure. Also relay switching devices 28 of Fig. 1 are not .shown in Figs. 2 and 3 but'the necessary' arrangements, including appropriate wiring, can, of course, readily be inserted in accordance with the diagram of Fig. l by one skilled in the art.

In addition to aiording increased radiation the inclusion of nine crystal groups in each row broadens the radiation or response pattern of the device of Figs. 2 and 3'in theplane of the row. Since the device will normally employ its prismatic properties in the horizontal plane, the broadening just mentioned will `occur in the vertical plane. For submarine detection work this is desirable as it will compensate forthe roll of the vessel Vupon which the radiator." or receiver is carried.,

For high power submarine radiators. i5-degree lY-cut Rochelle salt piezoelectric crystals, or some cut of Rochelle salt crystal approximating the 45-degree Y-cut, oifer substantial advantages as described in my above-mentioned copending application Serial No. 413,429. As mentioned above,

the practicable maximum power output limitfor crystal radiators is a point below that at which c'avitation begins to take place. For a crystal submerged in an ordinary liquid,` such as kerosene, for example, it has been discovered that cavitation will begin to occur at acoustic pres- 42 and casing-52. If transmitted to adjacent rows l such energy can impair or destroy the desired directive eilects by introducing energy of other than the desired phase and if .transmitted to the mounting plate and oase it can result in substantial dissipation and in the radiation or reception of energy from the sides or rear of the device, thus again imrairing the eiiiciency and the directive properties of the device. For a well balanced assembly in which the backing blocks and crystals are accurately proportioned the fourteen mounting strips 45 can be replaced by a single mounting plate and the backing blocks 44 for each row can be replaced by a single bar running he length of the row, thus eliminatinggassures of .85 to .90 atmosphere. When submerged in a highly viscous liquid, such, as castoroil for example; it has been discovered that cavitation will not begin until an acoustic pressure in excess of five atmospheres has been reached. The adsorbed water of the crystal surfaces should, of course, be carefully removed and the castor oil or other highly viscous medium should be carefully dried as explained in my above-mentioned cop'ending application Serial No. 413,429.

Since the power is proportional to the square of the acoustic pressure, the output power ca'- pacity of the crystal which can be realized withv out destruction of the crystal is increased in the order of twenty-live times by immersing it in a highly viscous liquid. I

The greatly increased power which may be radiated with crystals immersed in a highly viscous liquid probably results from the sluggishness of the liquid, which hows so slowly that no cavities form duringthe short intervals in which negative pressure exists.

The increased power capacity'of the individual 'crystal thus realized may be employed to 'adkets 8 and reducing the' amount of milling work required on the backing blocks.

Casing 52 should provide' adequate clearance between backing blocks 44 and the filter case 54 as well as the casing 52 itself so that substantially no energy will be lost or radiated in undesired directions from the sides orthe rear of the assembly. VIn exceptional cases. the casing 52V may be evacuated to prevent the transmission of com-f pressional wave energy across it. As pointed out vantage in constructing compressional wave radi--v ators which will transmit in the order of twentyve times the power of prior art radiators of like physical dimensions or it may be employed to obtain a given power radiation with a greatly reduced number of crystals and a much smaller over-all radiator structure than is required with prior art devices. f v

It is important that the cap enclosing the crystals provide a-suiilcient volume and a crosssectional area of the viscous fluid within it parthe reception of energy through the sides' or 4rear of a directional receiving device is particularly undesirable in submarine detecting systems-for tlallyas allel to the radiating surfaces of the crystal. The cross-sectional area should expand substanthe distance i'rom the radiating surfaces is increased, so as to"spread the power to an extent such that the acoustic pressure transmitted to the water in contact with the cap is somewhatfless than .an atmosphere. If effective4 spreading of the power is not' realized. cavitation in the water with consequent loss of power will take place adjacent the outer surface of the cap and the efiiciency oi' the 'radiator can be seriously impaired. This requirement of spreading the power is more readily satised for radiators of the type described in my copendingjapplication Serial No. 407,457, led August 19, 1941, entitled Radiating systems, Patent No. 2,411,551, in which, to reduce minor-lobe radiation (i. e., radiation at angles other than that of maximum radiation) the more central units Y to be transmitted in the viscous uid. Furthermore, the fact that the successive rows of crystals are driven in diiering phase relation, as described above, reduces the maximum eiective pressure at the surface of contact with the sea water. v

`In radiators oi the above type in which the successive groups of radiating elements are connected at corresponding points of successive sections of a wave filter, respectively,in order to obtain the particular desired distribution of the radiated power between the successive groups of radiating elements the impedance of the successive iilter sections can be adjusted. For the purposes of this specicaticn this process is designated as tapering the' iilter impedance and a lter so adjusted is designated as a tapered ilter. For example, in the radiator illustrated in Figs. 1, 2 and 3, if it is desired to drive each of the fourteen groups of crystals by substantially equal amounts of power. it is necessary to compensate' for the attenuation in the filter structure and the absorption of power by the successive radiating groups as power is transmitted from the input terminals 32 toward the terminating resistance 26. The diminution of energy is,

8 sired. for instance, to obtain smaller minor-lobe radiation. the impedance of the successive sections should increase more rapidly than above from the input end to the central un'it and then either remain substantially the same or even decrease again to the end or terminating unit. depending upon the power distribution desired. The principles involved are, of course, those discussed in my'above-mentioned copending application on Radiating systems. Serial No. 407,457, coupled with the well-known principle that the power which a load of a given impedance will absorb is a function of the impedance of the circuit from which the power is to be drawn.

The above arrangements are preferred illustrative embodiments of the principles of the invention. Numerous other arrangements within the spirit and scope of the invention will readily occur to those skilled in the art. For example, while the above illustrative embodiments employ groups of piezoelectric vibrators, it is obvious that magnetostrictive, electromagnetic or other of course, principally a function oi the dissipation of the lter sections and the absorption of power by the successive crystal groups in the sequence. In a particular model it was found satisfactory to increase the impedance of each section by approximately ve per cent of the impedance of the preceding filter section to provide substantially equal power radiation from all fourteen rows of crystals. In this instance the impedance of the first section was 6000 ohms and the impedances of the successive sections were 6,320 ohms, 6,710 ohms, 7,100 ohms, 7,600 ohms, 8,140 ohms, 8,770 ohms, 9.480 ohms, 10,320 ohms, 11,350ohms, 12,600 ohms, 14,130 ohms, and 16,100 ohms, respectively.

Of course,A if a distribution of power in which vibrating members could be substituted thereforv .an envelope enclosing said vibrating element said envelope being lled with a liquid oi' suiliciently high viscosity that the phenomena of cavitation Wilt not take place on the radiating surfaces of the vibrating element at the acoustical pressures developed, the proportions of said envelope being suillciently large that the acoustical pressure developed at its surface is less than one atmosphere.

` WARREN P. MASON.

REFERENCES CITED :The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 768,570 Mundy Aug. 2 3, 1904 1,067,207 Williams July 8, 1913 2,138,036 Kunze Nov. 29, 1938 2,248,870 Langevin July 8, 1941 1,117,766 A Berger Nov. 17, 1914 2,147,649 Haines Feb. 21, 1939 1,980,171 Amy Nov. 13, 1934 2,169,304 Tournier Aug. 15, 1939 768,573 Mundy Aug. 23, 1904 2,086,891 Bachmann et al. July 13, 1937 FOREIGN PATENTS Number Country Date 613,799 France Nov. 29, 1926 620,484 Germany Cot. 22, 1935 304,173 Great Britain Feb. 14, 1929 

